Hi, I’m the author of the previously mentioned extension.
First of all, great to see the extension is useful for you!
However, due to a missing API in Firefox, the manipulation is indeed more of a workaround in a hackish way and certainly not a good way. This is also stated in various places, so read the Readme for a more detailed explanation. The API Bugzilla request is here in case you want to upvote it.
Apart from all that recent Firefox versions, however have introduced a feature for developers so you can change the style in dev-tools. Just enable the option devtools.inspector.color-scheme-simulation.enabled and then you can find this button:
There was also a preference ui.systemUsesDarkTheme for globally forcing the dark style, but somehow that did not work for me in Firefox 83.